Author: Avanthi Chikka

Publisher:

Published: 2011

Total Pages: 127

ISBN-13:

Lumbar interspinous spacers have recently become popular as an alternative treatment for low back pain. These devices are primarily used to treat spinal stenosis and facet arthritis and are intended to unload the facet joints, restore foraminal height and provide stability mainly in extension while allowing normal range of motion in other loading modes at the index level and also at the adjacent segments. The goals of this study were three fold: (i) to evaluate the biomechanical stability provided by the Superion Interspinous Spacer (ISS) (Vertiflex®, San Clemente, CA) implanted in the lumbar spine; (ii) to study the effect of transection of supraspinous ligament (SSL) on the lumbar spine implanted with ISS; and (iii) to investigate the effect of graded facetectomies (50%, 75%, 100% facetectomies for both unilateral and bilateral cases) following the placement of ISS in the lumbar spine. This study is basically divided into two parts: an in vitro investigation and finite element analysis. The in vitro biomechanical study was conducted on six human lumbar motion segments (3 L2-L3 and 3 L4-L5) in the following test sequence: (i) intact, (ii) implanted (SSL intact), (iii) SSL dilated longitudinally at the center (with ISS), (iv) 50% resection of SSL (with ISS), (v) 100% resection of SSL (with ISS) (vi) injured (ISS removed). A finite element (FE) analysis was performed for the same test cases and also to investigate the effect of graded facetectomies using an experimentally validated 3D L3-S1 model. A bending moment of 10 Nm was applied in all loading modes (flexion-extension, right/left lateral bending, and right/left axial rotation) while a bending moment of 10 Nm with 400N compressive follower load was applied only in flexion-extension. Range of motion (ROM) was recorded for each of the test constructs. In addition to ROM, intradiscal pressure (IDP), facet loads and stress contour plots for these test constructs were obtained from the FE model. Repeated measures one way ANOVA was used to perform statistical analysis to determine the statistically significant differences between different test constructs for the in vitro data. The in vitro results showed that the mean ROM was significantly (p0.05) reduced in extension post-implantation. There was a minimal decrease in mean ROM in flexion as well, but it was not significant (p0.05). ROM in lateral bending and axial rotation were not affected. Also, there was no significant difference in ROM in any of the loading modes for the instrumented cases with and without SSL. The FE results were in agreement with the in vitro results except in flexion. Unlike the in vitro results, the flexion ROM increased slightly with progressive transection of SSL. From the FE data, it was observed that there was a significant reduction in IDP and facet loads in extension following the placement of ISS. However, there was no considerable difference in IDP and facet loads for the implanted cases with partial or complete transection of SSL. ROM, IDP and facet loads were not affected at the adjacent levels in any of the loading modes for any of the test constructs. It was also observed that there was an effect of graded facetectomies (50%, 75% and 100%) on the instrumented spine with a significant increase in ROM in flexion in case of both unilateral and bilateral facetectomies. The ISS provided an increased stability in extension while preserving motion comparable to intact in the other loading modes. Also, it can be inferred that SSL plays an insignificant role in segmental stability in extension, lateral bending and axial rotation and has nominal effect in flexion. In addition, the results suggest that ISS may not be used in combination with graded facetectomies for both unilateral and bilateral cases.